The present invention relates to a key telephone system, and more particularly a key telephone system with a data transmitting and receiving circuit provided in each terminal equipment or key telephone set coupled to a main unit, and is extremely advantageous when applied to a system, e.g. a key telephone system, to effect data transmission with a data signal having a substantially AC waveform being superimposed on a DC signal.
In key telephone systems comprising a main unit and a plurality of key telephone sets, each key telephone set is connected to the main unit through a pair of speech lines and a set of signal lines. Such systems are adapted to supply electrical power from the main unit to each key telephone set via the signal lines or both the speech lines and signal lines and to superimpose a data signal having a substantially AC waveform on a power supply voltage to effect transmission of the data signal by using the signal lines. In the prior art, it is known to employ a pulse-like signal varying in a negative direction to the data signal and to superimpose the pulse signal on a DC voltage applied to the signal lines.
However, as seen from FIG. 11A showing a waveform on the signal lines, a DC voltage E gradually lowers according as the superposition of a data signal Sd progresses and gradually rises from a time at which the application of the data signal Sd is removed. Such signal changes repeat, with the result that the power supply voltage changes. In addition, as seen from a partially enlarged view shown in FIG. 11B, a reference voltage Vs for detection used in a receiver circuit is fixed in its level, resulting in an unstable detection when the DC voltage is lowered. The drift of DC voltage E is due primarily to the distributed capacitance of the signal lines and transformer characteristics. The drift is particularly severe in systems employing "phantom" power supplies, wherein pairs of transmission lines are maintained at different relative potentials.
For stabilizing such an unstable detection, means as disclosed in Japanese Patent Publication No. 59-37612 and Japanese Patent Application Laid-open No. 59-91765 are already known.
However, the means disclosed in these prior art publications is only effective for a two-level or binary signal. Where an AC signal is superimposed on a DC voltage to change the DC voltage in both positive and negative directions, there occurs a need to handle a three-level or trinary signal. The means disclosed in the above-mentioned prior arts are not practically acceptable for the purpose of detecting such a three-level signal as a two-level signal.